1. Field of the Invention
The present invention relates to a process for obtaining purified acrylic acid from a crude aqueous acrylic acid, wherein the polymerization of acrylic acid is prevented. More specifically, the present invention relates to a process for obtaining highly-purified acrylic acid through distillation, wherein the polymerization of acrylic acid which tends to be caused in a distillation column is prevented when impurities having a lower boiling point, such as water and acetic acid, are removed by using an entrainer from a crude aqueous acrylic acid which is obtained from the catalytic oxidation of propylene and/or acrolein with molecular oxygen, conducted in a vapor phase.
2. Related Art
When a gas resulting from the vapor-phase oxidation of propylene and/or acrolein with a molecular-oxygen-containing gas, carried out in the presence of steam by the use of an oxidation catalyst is cooled and/or absorbed in water, a crude aqueous acrylic acid can be obtained. This crude aqueous acrylic acid contains not only acrylic acid but also some by-products such as acetic acid, formic acid, formaldehyde and acetaldehyde. When the degree of conversion of the oxidation reaction is low, the crude aqueous acrylic acid contains a small amount of unconverted acrolein used or formed. In such a case, the acrolein is removed from the crude aqueous acrylic acid by means of stripping or the like, and the resultant is subjected to purification to obtain purified acrylic acid.
The concentration of acrylic acid in the crude aqueous acrylic acid thus obtained is from 40% to 80% by weight. Among the by-products, the most important is acetic acid, and the crude aqueous acrylic acid contains from 1% to 5% by weight of acetic acid. It is not efficient to directly separate water, acetic acid and acrylic acid in the crude aqueous acrylic acid by distillation because of their chemical similarity and physicochemical properties such as vapor-liquid equilibriums. With respect to a method for obtaining purified acrylic acid, there have been proposed, in recent years, a number of processes which comprise the step of efficiently removing water by azeotropic distillation with an entrainer used, and the step of separating acetic acid by distillation. According to our classification, there are two types of processes for separating water and acetic acid, that is, a process which may be called "one-column process", wherein water and acetic acid are simultaneously removed from acrylic acid by using only one distillation column (Japanese Patent Publications Nos. 18967/1971, 20372/1971, 22456/1971, 34692/1971 and 21124/1974, and Japanese Laid-Open Patent Publication No. 246941/1993), and a process called "two-column process", in which dehydration and the separation of acetic acid are separately conducted by using two different distillation columns (Japanese Patent Publications Nos. 15569/1966, 18966/1971, 25451/1975 and 10691/1988, Japanese Laid-Open Patent Publication No. 181440/1991, and Japanese Patent Publications Nos. 15495/1994 and 15496/1994). The present invention concerns improvements in the two-column process.
In the former process, that is, in the one-column process, water and acetic acid are simultaneously removed by the use of only one column. It is therefore necessary to use a distillation column having a large theoretical number of plates, and a high reflux ratio is required also. When a distillation column having a large number of plates is used, the pressure at the bottom of the column may as the result be excessively high, so that it may be difficult to keep the temperature of the bottom product low. The use of such a distillation column is thus disadvantageous from the standpoint of energy consumption. In addition, acrylic acid is a material which is polymerized very easily, so that it is a fatal shortcoming from the viewpoint of prevention of the polymerization of acrylic acid that the bottom of the column reaches an excessively high temperature.
In the two-column purification process, water and acetic acid are respectively removed from acrylic acid by using two different distillation columns designed respectively therefor. Therefore, a column optimized and suitable for dehydration and one optimized and suitable for the separation of acetic acid can be respectively used. This process is thus advantageous as a whole from the viewpoint of energy even though two columns are used. Further, this two-column process is also advantageous in that acetic acid, which is a main by-product of the oxidation reaction of propylene and/or acrolein, can be separated and recovered from the overhead product of the acetic-acid-separating distillation column.